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How do I draw a graticule in GIMP?

I want to draw a graticule (latitude / longitude lines) on a map I'm making using GIMP (mostly). To keep it simple, I'm using a cylindrical projection, which means of course all the lines are straight lines. The trick is, the parallels are not all equally spaced and the equator is not in the center of the map. How do I draw a straight line in Gimp precisely where I need it to be?

Backwards way, but still works

Instead of making it a line, you can use a rectangular selection. Just make a rectangular selection of any size anywhere on the image, and then edit the location on the GIMP tool tab. You will see Mode, feathered selection, anti-aliasing, and a few other things. Under those, you will see "Position" Choose the X and Y location for the start of the line, and then the X and Y size (presumably the width of the image for X, and the line thickness for Y) CHoose the paintbrush tool, select whatever color you want to use, and then just paint inside the selection. Be sure to select "Fill Whole Selection" option if you already have different colors within the selection.

If you make the grid on a separate layer, you won't muck up your original image.

I played with it this morning, and found an answer that works for this problem. If you zoom in to 1600%, you can locate points to a fraction of a pixel. There's a scale that appears along the left and upper edges that tells you exactly what coordinate you're at - if I'd noticed that earlier I wouldn't have bothered you all with the question! . A little quality time with a spreadsheet told me where I wanted the lines, and then I used the path tool to place them precisely. If I was using a projection with curved lines, I could, in principle, do the same thing, though it might become tedious.